Aerogel is a solid with low density and high porosity obtained by removing the solvents and water within a nanopore network structure, wherein, the nanopore network structure consists of colloid particles or high polymer molecules coalescing with each other. Common SiO2 aerogel is a high dispersive solid nano material consisting of a SiO2 network frame and gas filling nanopores. The aerogel as an important product is widely known by scientists around the world, and the scientists around the world discovered numerous new features beyond imagination after decades of research. The aerogel has the features of low density, extremely high specific surface area, high porosity, etc., has uniquely excellent properties in mechanics, acoustics, thermotics, optics, chemistry, physics, and has a wide range of applications in the fields of thermal insulation materials, sound insulation materials, infrared rayabsorbing materials, catalyst materials, environmental protection materials, etc. However, the industrial scale production of the aerogel is limited by the complicated preparing process and the expensive drying apparatus. At present, the application of a small amount of aerogel prepared by employing supercritical drying apparatus and expensive cost is only limited to high-end uses such as military industry, aerospace, petroleum, sports accessories and so on.
The prior process technology for preparing aerogel is mainly divided into two parts, that is, the preparation of gel and the drying of the gel. As exemplified by the preparation of silicon dioxide aerogel, the silicon dioxide gel to be dried is prepared by using a silicon source, a solvent and a catalyst by the steps of hydrolysis, gelation, aging and solvent displacement; and then the aerogel is obtained by drying the gel through the method of supercritical carbon dioxide drying.
The most difficult problem during the preparation of the aerogel is to remove the water and solvents within the gel in case the structure can not be destroyed, whereas the bearing force of the ultra thin framework is extremely limited, and it is very easy for the pores to be collapsed and cracked by heating and drying. The collapse of the aerogel during drying is due to a capillary effect generated by its own structure during drying. The pores within the gel are similar to the capillaries. During drying, when a part of water and solvents within the gel are discharged out of the gel, a three-phase interface will appear, and the surface tension of the interface encourages the liquid in the interior to generate a concave surface in the capillary. The concave surface is generated by the surface tension, and this force is an inward force and tends to make the wall of the capillary collapse inwards. It is just used for preventing the forming of the surface tension by employing the supercritical method for drying. Common methods are ethanol and carbon dioxide supercritical drying methods. The ethanol supercritical drying requires pressure reaching 6.38 MP and high temperature at the same time. The carbon dioxide supercritical drying requires pressure reaching 7.38 MP. The vapor-liquid interface disappears when the ethanol or carbon dioxide is in a supercritical state, hence, the three-phase interface disappears, and there will be no surface tension. At this moment, a large number of carbon dioxide fluid or ethanol fluid displaces the water and solvents within the gel, then the carbon dioxide or ethanol is removed by reducing pressure, and the dried aerogel is obtained. However, the supercritical drying apparatus is a high pressure apparatus, and the use thereof is dangerous and expensive, so that the prepared product is expensive with poor yield at the same time. There is also an attempt to prepare the aerogel by employing the normal pressure method of heating or microwave. When the heating or microwave drying method is employed, it is difficult to accurately control the temperature, the temperature in the colloid is different according to the different distance from the heating source, the thickness of the colloid is not consistency and so on, so it causes that a large number of solvents and water existed within the colloid at different positions generates different vapor pressure due to different temperature. At the position with high temperature, the outward expansive force caused by the high vapor pressure will be larger than the capillary force, and the crack of the pores of the gel appears. At the position with low temperature, the vapor pressure will be low, and the outward expansive force caused by the vapor pressure is smaller than the capillary force, thus, the collapse of the pores of the gel appears. This heating or microwave drying method results in that the pores of the gel are cracked and collapsed, and the obtained silicon dioxide solid product is powdered. The collapse and crack of the pores cause that the dried product does not have insufficient pores, but has a large specific weight and poor thermal insulation, and does not have the characteristic of the aerogel. The bulk aerogel can also be obtained by the method of enhancing the gel strength; however, all the products obtained have high density and low porosity, and do not have excellent thermal insulation effect. Hence, the bulk aerogel with aerogel characteristics can be obtained only by removing the solvents and water within the aerogel in case of maintaining the aerogel with the three-dimensional pore structure and high porosity of the original gel.
If the supercritical drying method is not employed, the capillary force will appears when the three-phase interface appears during drying, and if the strength of the capillary wall can not bear this force, the capillary will be collapsed inwards. The aerogel prepared by drying can be the aerogel with excellent quality only by preventing the pores of the gel from being collapsed and cracked as much as possible.